Heat activated dry development of photoresist by means of active oxygen atmosphere
BACKGROUND OF THE INVENTION 
     The present dry development method constitutes an improvement of the dry development method disclosed and claimed in copending U.S. patent application, Ser. No. 752,040 filed Dec. 20, 1976, now abandoned. In the latter connection, dry development processing as contemplated herein and in the aforesaid U.S. application requires a photoresist material which may have a differential etchant rate of reactivity related to its selectively exposed areas and non-exposed areas, when subjected to an oxygen-containing gas in which monatomic oxygen and/or ozone are present as a reactive oxygen species wherein the differential rate of reactivity to an etchant treatment involving the reactive oxygen species is sufficiently large to enable the portion of the layer of photoresist material subject to the greater etchant rate to be completely removed from the underlying substrate surface before any substantial part of the remaining portion of the photoresist material is so removed. A desired pattern as determined by a latent image imparted to the layer of photoresist material through its selective exposure to a suitable energy source, such as ultraviolet radiation, X-ray, or E-beam radiation, is thereby developed in the layer of photoresist material by dry processing as contrasted to the conventional practice of employing liquid chemical solvents as selective etchants. 
     SUMMARY OF THE INVENTION 
     This invention is directed to a dry development method for producing a pattern in a layer of photoresist material which has been selectively exposed to an energy source such as ultraviolet, X-ray, or E-beam radiation, wherein development of the selectively exposed photoresist layer is accomplished in a manner constituting an improvement of the method disclosed and claimed in the aforesaid U.S. patent application, Ser. No. 752,040 filed Dec. 20, 1976, now abandoned. To this end, it is contemplated that the photoresist material will be of a character enabling a chemical or physical change to be caused therein after the formation of an image has been imparted thereto by the selective exposure of the layer of photoresist material to a suitable energy source. The chemical or physical change selectively occurs in either of the exposed or unexposed portions of the layer of photoresist material, thereby significantly enhancing selective removal of the photoresist material in developing the pattern provided by the image formed therein, wherein the development occurs by subjecting the selectively exposed layer of photoresist material to an oxygen-containing gaseous atmosphere including an active oxygen species which may be monatomic oxygen and/or ozone for differentially reacting the gaseous reactive oxygen species with the exposed and unexposed regions of the layer of photoresist material. In a preferred form of the method contemplated herein, the chemical or physical change imparted to either one of the exposed or unexposed portions of the photoresist layer induces volatization thereof such that one of the exposed and unexposed portions of the photoresist layer assumes a reduced thickness in contrast to the other of the exposed and unexposed portions. A heat treatment, for example, either accompanying selective exposure of the photoresist layer to a suitable energy source or as a separate stage following such selective exposure may accomplish this reduction in thickness when employed with a suitable photoresist material. 
     The area of reduced thickness of the layer of photoresist material are subject to a rate of removal by the gaseous active oxygen species greater than the etchant rate to which the remaining regions of the photoresist material are subject, with the reduced thickness imparted thereto enhancing the differential reaction as between the exposed and unexposed regions of the layer of photoresist material, thereby enabling the regions of photoresist material of reduced thickness to be completely removed from the substrate surface to form the desired pattern in the layer of photoresist material from the remaining portions of photoresist material. 
     More specifically, it is contemplated that the photoresist material employed may be of the type subject to heat-volatilization in its unexposed state such that the application of a heat treatment thereto, after the layer of photoresist material has been selectively exposed to an energy source for imparting an image therein, causes the photoresist material in the unexposed areas to be partially volatized, thereby characterizing the unexposed areas of the layer of photoresist material as having a thickness of reduced dimension as contrasted to the exposed areas of the layer of photoresist material. Such a photoresist material is of the so-called negative type and may be based upon polyvinyl acetate, a commercial form of which is known as Horizon U-200 available from Horizons Research Incorporated of Cleveland, Ohio. 
     The advantages of the invention will be better understood from the following description when read in connection with the accompanying drawings in which: 
    
    
     FIG. 1 is a cross-sectional view showing an initial stage in the method of dry developing a pattern in a photoresist layer as applied to a substrate in accordance with the present invention; 
     FIG. 2 is a cross-sectional view similar to FIG. 1, but showing a subsequent stage in the method following selective exposure of portions of the photoresist layer to an energy source, such as ultraviolet radiation, X-ray, or E-beam radiation, to impart a latent image therein; 
     FIG. 3 is a cross-sectional view showing a stage in the method subsequent to that shown in FIG. 2 following application of a heat treatment to the photoresist layer; and 
     FIG. 4 is a cross-sectional view showing the final stage in the method following complete development of the latent image in the photoresist layer by subjecting the photoresist layer to an oxygen-containing gaseous atmosphere in which monatomic oxygen and/or ozone are present as a reactive oxygen species. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with this invention, a dry development method constituting an improvement of the method disclosed and claimed in pending U.S. patent application, Ser. No. 752,040 filed Dec. 20, 1976, now abandoned, referred to earlier, is disclosed, wherein portions of a photoresist layer are selectively removed from the surface of a substrate so as to produce a pattern in the layer of photoresist material for subsequent use in performing various treatments on the exposed regions of the substrate therebeneath. FIG. 1 illustrates a substrate 10 of semiconductor material such as silicon, for example, to which a layer 11 of insulation material has been applied. The insulation material of layer 11 may be silicon dioxide. A layer 12 of photoresist material is then applied to the insulation layer 11. It will be understood that the photoresist layer 12 may be applied directly to the surface of the substrate 10 in a given instance where it is desired to perform subsequent treatments, such as diffusion of dopant materials, or deposition of electrical contacts or other semiconductor components, for example, on selected surface portions of the substrate 10. In this respect, the desired pattern is originally imparted to the layer 12 of photoresist material by selectively exposing the layer 12 of photoresist material to an energy source, such as ultraviolet light, X-ray or E-beam radiation, to produce a photographically defined latent image of exposed regions 13 and unexposed regions 14 therein (FIG. 2). 
     In accordance with the present invention, the photoresist material which is employed must be of a type subject to a chemical or physical change upon undergoing exposure and/or another treatment thereto, wherein a selective thickness reduction of areas in the photoresist material conforming to the latent image imparted thereto can be achieved. In this context, the ability of a photoresist material to undergo a selective relative thickness reduction in areas comprising one of the exposed and unexposed areas of the photoresist material as contrasted to the other of such areas is critical to the present method. 
     By way of a specific example, the selective exposure of a layer of photoresist material to a suitable energy source in the manner previously described provides both exposed and non-exposed portions 13 and 14 in the photoresist material 12 defining the latent image imparted thereto (FIG. 2). It will be observed in FIG. 2 that the unexposed region 14 has acquired a slight thickness reduction with respect to the exposed regions 13, the thickness reduction of the unexposed region 14 resulting from partial volatilization thereof due to induced heat from the energy source over the entire surface of the layer of photoresist material. The exposed portions 13 are polymerized from direct exposure to the energy source and therefore become resistant to volatilization. Upon subjecting the selectively exposed photoresist material 12 to a heat treatment, the unexposed portion thereof undergoes partial volatilization so as to assume a reduced thickness 14&#39; (FIG. 3) as contrasted to the exposed portions 13 of the photoresist material 12. One such photoresist material having the aforesaid properties is based upon polyvinyl acetate, known as Horizon U-200 and commercially avaialable from Horizons Research Incorporated of Cleveland, Ohio. Substantially similar photoresist materials based on polyvinyl acetate and available under the names &#34;PHOTORIST&#34; and &#34;AQUARIST&#34; from the said Horizons Research Incorporated are also suitable. A photoresist material of this character, wherein the dimensional thickness of selected portions thereof can be changed by processing prior to the development step effectively enhances the differential rate of reactivity existing between the exposed portions and non-exposed portions of the photoresist material as employed in a dry development process of the type disclosed in the aforesaid U.S. patent application, Ser. No. 752,040, filed Dec. 20, 1976, now abandoned. This difference in thickness across the layer of photoresist material depending upon whether the areas of the photoresist material are exposed or unexposed facilitates dry development in that the complete removal of the areas of photoresist material of reduced thickness down to the substrate surface is readily accomplished before degradation of the remaining portions of the photoresist material occurs to a degree damaging the pattern imparted thereto as illustrated in FIG. 4. Thus, it will be understood that the differential reaction rate as between the exposed and non-exposed areas of the photoresist material is significantly enhanced by treating the photoresist material prior to the development step in a manner causing a selective thickness reduction in the areas thereof depending upon whether such areas are exposed or non-exposed areas. 
     One type of photoresist material subject to this effect is polyvinyl alcohol (PVA) as activated with ammonium dichromate. A typical photoresist formulation can be made from a dry, bulk polyvinyl alcohol, such as that sold by Monsanto Corporation of St. Louis, Mo., under the name Gelvatol 20/30. Water solutions of PVA contain about 10% solid PVA. The activating chromate solution is generally ammonium dichromate and is usually prepared as a 10% water solution. An activated resist solution can be made by mixing 50 ml 10% PVA with 4 ml 10% ammonium dichromate. This photoresist material selectively undergoes a thickness reduction during the exposure thereof, wherein the areas unexposed to the energy source, such as ultraviolet radiation, partially volatize to assume a thickness of reduced magnitude. The photoresist may be prepared by dissolving polyvinyl alcohol in water to form a clear solution, following which trace quantities of a chromate solution, such as ammonium dichromate, are added to make the resulting solution photosensitive. Typically, such a photoresist is developed after the selective exposure thereof by applying water which redissolves the unexposed areas. However, in accordance with the present invention, the photoresist material is dry developed to produce the pattern therein as imparted by its selective exposure to a suitable energy source. To this end, the layer of photoresist material is exposed to a gaseous reactive oxygen species, as for example, an oxygen plasma (i.e. monatomic oxygen) maintained at 1/2-1 torr O 2 , RF 12.5 MH z  plasma for 3-4 minutes. Although this is a useful embodiment of a photoresist material suitable for practicing the present dry development method, the unexposed areas of the photoresist material present some difficulty with respect to the completeness with which such areas may be cleared to developed out, since the gaseous reactive oxygen species to which the photoresist material is subjected tends to polymerize the otherwise unpolymerized photoresist. 
     A preferred photoresist material for use in practicing the method in accordance with this invention and of the type which selectively changes thickness dimensions during processing is the previously mentioned photoresist material Horizon U-200. This photoresist material appears to have a polyvinyl acetate base, thereby being chemically similar to polyvinyl alcohol (PVA). After being selectively exposed to a suitable energy source, this photoresist material undergoes a 40% to 60% thickness reduction in its non-exposed areas upon being subjected to a heat treatment maintained at a magnitude of approximately 90° C. for 1-2 minutes. It is believed that this selective thickness reduction occurs because components of the photoresist material in the unpolymerized areas (or non-exposed areas) are volatized by the heat treatment, whereas these components are chemically bound by the polymerization thereof in the selectively exposed areas. Thus, the heat activated thickness reduction occurs only in the unpolymerized areas or non-exposed areas of this photoresist material. 
     It will be understood that the family of photoresist materials capable of selective thickness reduction which may be suitable for use in practicing the method according to the present invention could be formed from several types of components. Where the photoresist material is of the negative type such that it would be rendered less soluble on exposure to a suitable energy source such as electromagnetic radiation, wherein such exposure would provide a negative or reverse pattern of the mask through which the electromagnetic radiation is directed, it is contemplated that the photoresist material composition may comprise monomers, which can be the same or different monomers, of a character forming cross-linkages upon exposure to the energy source so as to form a higher temperature stable polymer. In the unexposed areas, therefore, the monomer or monomers are unchanged or unpolymerized, and can be volatized by a moderate heat treatment. In a so-called positive photoresist material, the photoresist material becomes more soluble upon being exposed to electromagnetic radiation, thereby providing a positive image of the mask through which the radiation was directed. Thus, a positive photoresist material for use in the present method may comprise a composition of a material dissociated by light when exposed to a suitable energy source to form components of which either one or both may be volatized by applying a moderate heat treatment. 
     It is further contemplated that a photoresist material may be provided with a composition which is not included in the reaction involving the basic cross-linking or de-cross linking as brought about by the exposure of the photoresist material to a suitable energy source, but which serves to prevent a heat-activated thickness reduction of the photoresist material. This composition would be preferentially retained in either the exposed (cross-linked) or unexposed areas of the photoresist material upon subjection of the photoresist material to an appropriate heat treatment, thereby achieving selective removal of the composition and enabling the heat-activated thickness reduction of the areas of photoresist material from which such composition has been removed. 
     Thus, it will be understood that by selectively causing predetermined portions of the layer of photoresist material to undergo a chemical or physical change resulting in a reduction in thickness thereof, either by the application of a heat treatment to the layer of photoresist material or in other ways, the dry development differential reactivity is significantly enhanced, such that either the exposed or the unexposed areas of the photoresist material can be completely removed (depending upon whether the photoresist material is of a positive or negative type) before the other areas undergo sufficient degradation from the dry development procedure to damage the developing pattern. It will be further understood that a photoresist material having the property of increased selective thickness reduction when subjected to processing prior to the development thereof proportionately lessens the degree of dry development differential reactivity required as between the exposed and non-exposed areas of the photoresist material for producing the desired pattern. 
     In the dry development process of this invention, a layer of photoresist material having the selective thickness reduction characteristics herein described is applied to a substrate of different material, and thereafter selectively exposed to an energy source of a suitable type to produce a latent image therein. In one form of the invention, the selective exposure alone is sufficient to cause a thickness reduction of the photoresist material in one of the exposed and non-exposed areas thereof as contrasted to the other of the exposed and non-exposed areas. In such an instance, a separate heat treatment to achieve partial volatization of selected areas of the photoresist material with resulting selective thickness reduction may not be required. However, in a preferred embodiment of the invention, the substrate or slice as coated with the photoresist layer is heated and then contacted with a gaseous atmosphere including a gaseous reactive oxygen species adapted for differentially reacting the gaseous oxygen species with the exposed and unexposed regions of the layer of photoresist material. 
     By way of example, the gaseous reactive oxygen species may be provided by an oxygen plasma derived from a mixture of oxygen and argon, from which monatomic oxygen may be generated as a plasma species by the use of electrical fields or gaseous discharges at pressures below atmospheric pressure through known procedures. The layer of photoresist material having areas of reduced thickness as contrasted to other areas, wherein the reduced thickness areas can be either one of the exposed or non-exposed portions of the photoresist material, is exposed to the gaseous reactive plasma for differentially reacting the gaseous reactive plasma with the exposed and non-exposed regions of the photoresist material. Preferably, the areas of reduced thickness of the photoresist material are subject to a significantly greater etchant rate as contrasted to the etchant rate for the remaining areas of the layer of photoresist material such that the gaseous reactive plasma selectively attacks the areas of reduced thickness of the photoresist material for completely removing these areas of reduced thickness in a selective manner, with the remaining areas providing the desired pattern in the layer of photoresist material. In this manner, a dry development of the latent image in the photoresist layer is accomplished. 
     It will be understood that that latent image in the photoresist layer can be dry developed in accordance with the present method upon subjecting the photoresist layer having the areas of reduced thickness selectively disposed therein to a gaseous atmosphere which could be ozone or a mixture of monatomic oxygen and ozone, such as is disclosed in U.S. patent application, Ser. No. 752,040, filed Dec. 20, 1976, now abandoned. However, in the present instance, oxygen plasma containing monatomic oxygen is a preferred dry development source, since improved resolution of the pattern in the photoresist layer is attained by the use thereof. The oxygen plasma may be produced by employing an oxygen-argon mixture in a chamber between electrodes to which an RF generator is connected, wherein the RF generator operates at 13.5 MH z  in a pressure range of from 0.5 torr to 1 torr with the chamber being maintained at a temperature in the approximate range of 150° to 250° C. at which dry development takes place. 
     For the purpose of describing specific examples of the heat-activated dry development process according to the present invention, reference is made to the following. 
     EXAMPLE 1 
     An oxidized silicon slice was coated with a layer of Horizon U-200, a photoresist material commercially available from Horizons Research Incorporated of Cleveland, Ohio. The initial thickness of the photoresist layer was 5,000 Å. The photoresist layer was then selectively exposed to ultraviolet radiation to impart a latent image thereto. The photoresist layer was heated to about 65° C. for 1-2 minutes causing a reduction in the thickness of the non-exposed areas thereof. The slice with the photoresist layer was placed in a reaction chamber and exposed to a gaseous atmosphere including ozone in a 2-3% concentration of an oxygen-containing gaseous atmosphere for 30 seconds at about 250° C. A developed pattern of the latent image of the photoresist layer of the order of 4,000 Å in thickness remained on the slice. 
     EXAMPLE 2 
     An oxidized silicon slice was coated with a layer of Horizon U-200 photoresist material. The photoresist layer was then selectively exposed to ultraviolet radiation for approximately 15 seconds to impart a latent image thereto. The photoresist layer was then heated to 90° C. for 3 minutes. The total thickness of the exposed area of the photoresist layer after the 3 minute heat treatment was 8763 Å, and the thickness of the non-exposed area of the photoresist layer was 5789 Å after the 3 minute heat treatment, leaving a thickness differential between the exposed and non-exposed areas of 2974 Å. The slice with the photoresist layer was then placed in a reaction chamber and exposed to an oxygen-containing gaseous atmosphere including oxygen plasma at a pressure of approximately 1 torr with the chamber being maintained at room temperature and the photoresist layer on the slice being subjected to the oxygen plasma atmosphere for a period of 1 minute. The slice was then removed, and the remaining photoresist layer thereon was measured. The total thickness of the remaining photoresist layer in the exposed area thereof was 8255 Å, whereas the thickness of the photoresist layer in the non-exposed area was 4445 Å. Thus, the differential thickness between the exposed and non-exposed areas of the photoresist layer following the oxygen plasma treatment was 3810 Å, thereby demonstrating a developed pattern of the latent image in the photoresist layer. 
     EXAMPLE 3 
     An oxidized silicon slice similar to that employed in Example 2 was coated with a layer of Horizon U-200 photoresist material. The photoresist layer was then selectively exposed to ultraviolet radiation for 15 seconds to impart a latent image thereto. The photoresist layer was then heated to 90° C. for 3 minutes. After this heat treatment, the photoresist layer had a total thickness of 8255 Å in the exposed area thereof and a total thickness of 5207 Å in the non-exposed area, thereby having a differential thickness of 3048 Å. After subjecting the slice with the photoresist layer to an oxygen-containing gaseous atmosphere including oxygen plasma in a reaction chamber at a pressure of approximately 1 torr and at room temperature over a period of 4 minutes, it was observed that the photoresist material had been entirely removed in the non-exposed area down to the level of the oxide layer on the silicon slice, while the total thickness of the photoresist layer in exposed areas thereof varied between 3302 Å at a central portion of the slice to 2286 Å at the peripheral edge thereof. Complete removal of the photoresist material in the non-exposed areas thereof provided a developed pattern of the latent image initially imparted to the photoresist layer by the selective exposure thereof to ultraviolet radiation. 
     Although preferred embodiments of the invention have been specifically described, it will be understood that the invention is to be limited only by the appended claims, since variations and modifications of the preferred embodiments will be apparent to those skilled in the art.